Current commercial processes for separating acid gases such as CO.sub.2 and H.sub.2 S from gaseous mixtures containing these acid gases typically utilize chemical absorbents such as amines or aqueous salt solutions. A variety of amines, mostly alkanolamines, are used as CO.sub.2 absorbents to separate CO.sub.2 from mixtures containing other gases. Suitable alkanolamines which can be used for removing CO.sub.2 from gaseous mixtures include monoethanolamine, diethanolamine, diglycolamine, diisopropanolamine and triethanolamine.
The above-mentioned primary and secondary amines react with CO.sub.2 to form carbamates. Carbamates are relatively stable but upon heating, the CO.sub.2 can be desorbed thereby regenerating the CO.sub.2 -free absorbent. Typical heats of reaction of amines with CO.sub.2 are on the order of 10-20 kcal/mole CO.sub.2, which is relatively high. In contrast, tertiary amines do not form carbamates, but instead, generate hydroxide in aqueous solutions wherein the hydroxide subsequently reacts with CO.sub.2 to give HCO.sub.3.sup.- and/or CO.sub.3.sup.2-. Since the heats of reaction of CO.sub.2 with tertiary amines are less than those involving primary and secondary amines, less energy is needed to desorb CO.sub.2 from tertiary amine solutions.
Various processes have been developed for absorbing CO.sub.2 and H.sub.2 S which utilize solutions of strongly alkaline salts. The most commonly used solutions contain sodium and potassium carbonate, phosphate, borate, arsenite and phenolate, with potassium carbonate perhaps being preferred. Catalysts are known to increase the activity of alkaline salt solutions with respect to the rate of absorption and desorption of CO.sub.2.
Sterically hindered amines have been used to remove gases such as CO.sub.2 and H.sub.2 S from gaseous streams. As the structure of amines become more sterically hindered, the carbamate becomes increasingly less stable. Moderately hindered amines are characterized by high rates of CO.sub.2 absorption and high capacities for CO.sub.2 while severely hindered amines are typically characterized by low rates of CO.sub.2 absorption and higher than standard capacities and selectivities for removing H.sub.2 S from gaseous streams containing CO.sub.2.
U.S. Pat. No. 4,973,456 issued Nov. 27, 1990 to Quinn et al. discloses a process for using salt hydrates as reversible absorbents to remove CO.sub.2, H.sub.2 S and other acid gases from gas streams. The absorption and desorption steps may be performed at the same temperature. Additionally, absorbed gas may be desorbed from the absorbent by lowering the gas pressure above the melt or by heating, although heating is not essential. The absorbent is a liquid at operating temperature and the gas is desorbed from the absorbent by lowering the CO.sub.2 pressure above the melt.
U.S. Pat. No. 4,944,934 issued Jul. 31, 1990 to Dunbobbin et al. discloses a process for separating oxygen from air by reversible chemical absorption at a significantly elevated temperature, e.g., 630.degree. C. to 870.degree. C., and desorption at a lower elevated temperature, e.g., 370.degree. C. to 500.degree. C. The chemical absorbent, which is utilized in a slurry state, is a vanadium bronze.